[es6] Function length property should be configurable

test/mjsunit/regress/regress-function-length-strict.js

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 19 matching lines @@
 "use strict";
 
 function foo(a, b, c) {
   return b;
 }
 
 var desc = Object.getOwnPropertyDescriptor(foo, 'length');
 assertEquals(3, desc.value);
 assertFalse(desc.writable);
 assertFalse(desc.enumerable);
-assertFalse(desc.configurable);
+assertTrue(desc.configurable);
 assertThrows(function() { foo.length = 2; }, TypeError);